drm/i915: Return the real error code from intel_set_mode()

Note: This patch also adds a little helper intel_crtc_restore_mode for
the common case where we do a full modeset but with the same
parameters, e.g. to undo bios damage or update a property.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
[danvet: Added note.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>